Drilling for natural gas, oil, or coalbed methane is conducted in a number of different ways. In conventional overbalanced drilling, a weighted mud system is pumped through a length of jointed rotating pipe, or, in the case of coiled tubing, through a length of continuous coiled tubing, and positive displacement mud motor is used to drive a drill bit to drill a borehole. The drill cuttings and exhausted pumped fluids are returned up the annulus between the drill pipe or coiled tubing and the walls of the drilled formation. Damage to the Formations, which can prohibit their ability to produce oil, natural gas, or coalbed methane, can occur by filtration of the weighted mud system into the formation due to the hydrostatic head of the fluid column exceeding the pressure of the formations being drilled. Damage may also occur from the continued contact of the drilled formation with drill cuttings that are returning to surface with the pumped fluid.
Underbalanced drilling systems have been developed which use a mud or fluid system that is not weighted and under pumping conditions exhibit a hydrostatic head less than the formations being drilled. This is most often accomplished by pumping a commingled stream of liquid and gas as the drilling fluid. This allows the formations to flow into the wellbore while drilling, thereby reducing the damage to the formation. Nevertheless, some damage may still occur due to the continued contact between the drill cuttings and exhausted pumped fluid that are returning to surface through the annulus between the drill string or coiled tubing and the formation.
Air drilling using an air hammer or rotary drill bit can also cause formation damage when the air pressure used to operate the reciprocating air hammer or rotary drill bit exceeds formation pressure. As drill cuttings are returned to surface on the outside of the drill string using the exhausted air pressure, damage to the formation can also occur.
Formation damage is becoming a serious problem for exploration and production of unconventional petroleum resources. For example, conventional natural gas resources are deposits with relatively high formation pressures. Unconventional natural gas formations such as gas in low permeability or “light” reservoirs, coal bed methane, and shale gases have much lower pressures. Therefore, such formations would damage much easier when using conventional oil and gas drilling technology.
Directional and horizontal drilling technology using a single coiled tubing drill string is known in the art. Thus, downhole tools useful for directional and horizontal drilling using coiled tubing are readily available. For example, coiled tubing drilling operations use existing technologies for directional measurement systems and orientation of the drilling assembly, but because such devices are being used with single strings of coiled tubing, drilling fluids are pumped down the coiled tubing and returned up the annulus between the coiled tubing and the wellbore wall.
In Canadian Patent # 2,079,071 and U.S. Pat. No. 5,215,151, issued to Smith and Goodman, incorporated herein by reference, a directionally drilling method is taught using coiled tubing which involves connection of a directional bottom hole assembly to a single string of coiled tubing. The directional bottom hole assembly is in electrical communication with existing directional drilling downhole sensors by means of an electric cable inside the coiled tubing. The downhole sensors are coupled with a device for orienting or rotating the bottom hole assembly by way of fluid pressure or fluid rate variations. This drilling technology can be used in underbalanced drilling operations.
U.S. Pat. No. 5,394,951, issued to Pringle et al, incorporated herein by reference, teaches a method of directional drilling with coiled tubing using a commercially available electrical steering tool, mud-pulse and/or electromagnetic measurement-while-drilling (MWD) equipment. Further, Canadian Patent No. 2,282,342, issued to Ravensbergen et al, incorporated herein by reference, defines a bottom hole assembly for directional drilling with coiled tubing which includes electrically operated downhole data sensors and an electrically operated orientor for steering capabilities while drilling.
Common to all the above referenced patents is the use of a single string of coiled tubing with a single path of flow within the coiled tubing. These patents further establish the existence of directional drilling capabilities on coiled tubing, with some reference to underbalanced drilling operations. The present invention extends the application of these existing technologies to concentric coiled tubing operations with reverse circulation of drill cuttings and formation fluids so as to avoid prolonged contact of these materials and associated damage with the formation. The present invention uses existing coiled tubing directional drilling technologies modified to provide for reverse circulation of the drilling medium and produced fluids.
The present invention reduces the amount of contact between the formation and drill cuttings which normally results when using air drilling, mud drilling, fluid drilling and underbalanced drilling by using a concentric coiled tubing string drilling system. Such a reduction in contact will result in a reduction in formation damage.